1. Field of the Invention
The present invention relates generally to the lubrication and hydraulic industry, and particularly to an apparatus and a process used for the removal of free, emulsified, or dissolved water from oil, and more generally, from liquids of low volatility.
2. Discussion of the Related Art
Oil is used in lubrication and hydraulic systems. It is widely recognized that the presence of water has deleterious effects on the oil in such systems, the components in the systems, and the operation of the systems. It is well known that corrosion; oil oxidation, chemical wear and tear, reduced bearing fatigue life, and loss of lubricity may result when water contamination enters a lubrication or hydraulic system. These deleterious effects can be directly attributed to water present in free, emulsified or dissolved form.
Consequently, significant efforts have been made to remove water from oil in order to provide optimal performance of lubrication and hydraulic systems. The devices and systems that have been used to remove water contamination include settling tanks or reservoirs, centrifuges, water absorbing filters, and vacuum dehydration oil purifiers. However, these have had significant limitations in either their water removal capabilities, ease of operation, capital costs, or operating costs, as will be discussed.
Settling tanks remove bulk quantities of xe2x80x9cfreexe2x80x9d water from oil based on the difference in their densities and gravitational settling. To be effective in removing xe2x80x9cfreexe2x80x9d water, settling tanks require large residence times and a significant amount of floor space. However, they are ineffective in separating oil-water emulsions and are not capable of removing dissolved water.
Centrifuges accelerate the gravitational settling of water from oil by imposing centrifugal force on the fluid that, in effect, elevates the gravitational force. Centrifuges are effective in removing free water from the oil. However, these centrifuges are generally expensive, and have limited capability of separating oil-water emulsions. They cannot remove dissolved water from the oil.
Water absorbing filters use special filter media that absorbs water from the oil. As the water is absorbed, the media swells, the flow is restricted, and the pressure drop across the filter rises. When the pressure drop reaches a predetermined level, the water absorbing filter is removed, disposed of, and a new filter is installed. These water-absorbing filters are effective in removing free water but have marginal effect in removing emulsified or dissolved water from the oil. In addition, water-absorbing filters have a limited capacity for water. Therefore, they must be replaced once they are saturated with water. Consequently, they are typically only used in applications where trace amounts of water are present. In applications where water concentrations are higher, the cost of continuously replacing water-absorbing filters becomes very high. Several types of vacuum dehydration oil purifiers have been used for oil dehydration. These generally operate under the principle of vacuum distillation, mass transfer of moisture from the oil to dry air, or a combination of the two.
In vacuum distillation, a vacuum is applied to reduce the boiling point of the water. For example, while the boiling point of water is 100xc2x0 C. (212xc2x0 F.) at 1013 mm H2O (29.92xe2x80x3 Hg) barometric pressure (standard atmospheric pressure), its boiling point at 100 mm H2O (approximately 26xe2x80x3 Hg of vacuum) is only 50xc2x0 C. (122xc2x0 F.). By applying a sufficient vacuum relative to the temperature of the oil, the water in the oil will evaporate from the oil into the low-pressure air (vacuum), thus dehydrating the oil.
Flowing the oil into a contactor vessel which has a vacuum applied to it by means of a vacuum pump is the typical means by which this is achieved. In order to maximize the water vaporization rate in a given vessel, large surface area-to-volume ratios of oil are preferred. This can be accomplished by means of flowing the oil over structured packing, random packing, cascading plates, spinning discs, or other methods well known in the vacuum distillation and contactor fields. The oil usually enters at the top of the contactor and flows gravitationally downward over the packing, spreading into relatively thin films. The oil collects in the bottom of the vessel where it must be pumped out by means of an oil pump. Examples of these are U.S. Pat. No. 4,604,109 by Koslow and U.S. Pat. No. 5,133,880 by Lundquist, et al. Heat may be added to the oil in order to reduce the amount of vacuum needed.
Vacuum is applied to lower the water boiling point, and to increase the water removal rate. Heat may also be applied to increase the water removal rate. However, great care must be taken in not applying too much heat and/or vacuum because more and more of the lower molecular weight hydrocarbons in the oil will also be vaporized as the temperature and/or vacuum is increased to levels below their boiling points. It should be understood that any liquid with a boiling point less than water will also be removed. This may, or may not be desirable, depending upon the application.
Mass transfer-based systems use similar contactor vessels. However, rather than relying on distillation for removal of the water, dry air or gas is continuously passed countercurrently upwards across the oil that flows downward. Water molecules in the oil will move via a concentration gradient into the relatively drier air. The now humid air is drawn from the contactor by a vacuum pump or blower and exhausted to atmosphere. It is not necessary to heat the oil more than the boiling point of water in order for the water to vaporize. Therefore, less heat and/or vacuum can be used for water removal with a mass transfer-based system than in vacuum distillation systems.
While vacuum distillation and mass transfer systems do remove free, emulsified and dissolved water, they have several drawbacks that have prevented their widespread use. In both systems, liquid level controls are used within the vessel in order to ensure that the oil level does not become so low so that the oil pump runs dry. The liquid level controls also function to ensure that the oil level does not become so high that the vacuum vessel fills with oil. This would reduce or eliminate the water removal efficiency of the vessel and may even lead to the oil entirely filling the vessel and overflowing into the vacuum pump.
Vacuum purifiers are also subject to foaming within the vessels as water is vaporized within the oil. This foam has a lower specific gravity than the oil and can cause malfunctioning of the liquid level controls and a reduction in the performance of the purifier.
Due to the very nature of the use of heaters, controls, pumps, etc., purifiers are relatively complex pieces of equipment. In addition, the type of packing used, the viscosity of the oil, and the airflow rate, limit the flow rates through contactor vessels. This usually results in very large vessels being used relative to the amount of flow. When packaged with all of the necessary oil pumps, vacuum pumps, heaters, controls, electrical panels and connections, the system becomes quite large and expensive. With the number of components and complexity of these systems, the maintenance and operating costs are usually quite high as well.
Due to their ability to remove free, emulsified or dissolved water from oil, vacuum dehydration oil purifiers have become the desired method for water removal from oil. However, the drawbacks associated with vacuum oil purifiers have prohibited these purifiers from being widely used and/or are not practical on the majority of lubrication or hydraulic systems. Because of their relatively large size and costs, they are limited to non-mobile, stationary applications, and are not practical for, use on mobile equipment.
Due to their high capital cost, they are typically not permanently installed in a system unless it is a relatively large, expensive lubrication or hydraulic system. Instead, they are usually shared by several systems by using one to purify the oil on one machine or reservoir for a period of time, and then move it to another machine, etc. However, when the purifier is being used in this manner, the oil in the machines that are not connected to the purifier can become contaminated with water. This oil will remain contaminated until the purifier can be reattached to them and the oil dehydrated again. Thus, those skilled in the art have continued to search for better ways to remove oil from water. Applicants have directed their efforts toward membrane based systems.
Membrane based systems have been used to remove water from organic systems. It must, however, be recognized that the presence of pores or defects in a membrane used for this purpose will result in the hydraulic permeation of the oil to the permeate side. This situation will result in the loss of oil. It will also allow the non-volatile oil to coat the permeate side of the membrane, thereby fouling the membrane and reducing its effectiveness in permeating water.
U.S. Pat. No. 4,857,081 to Taylor discloses a process for the dehydration of hydrocarbons or halogenated hydrocarbon gases or liquids. This process is based on a cuproammonium regenerated cellulose membrane. Cuproammonium regenerated cellulose membranes are known to those skilled in the art to have a structure of mutually connected passages or pores (U.S. Pat. No. 3,888,771 to Isuge et al). These membranes are also said to have a distribution of pores of the order of 10-90 xc3x85, with a mean of 30 xc3x85 (U.S. Pat. No. 3,888,771 to Isuge et al, U.S. Pat. No. 5,192,440 to Sengbusch). The mechanism for separation of water from the liquid organic phase through this cuproammonium regenerated cellulose is that of dialysis. The permeating species permeates the membrane as a liquid. Since the membrane has pores, it permits hydraulic permeation through it. Water-soluble species may permeate through it as well. This precludes its utility in the dehydration of oil, as the oil will always have a finite solubility in water.
Even if Taylor were satisfactory for dehydration of oil, the structure of Taylor will itself cause defects. The molecular structure of the regenerated cellulose membranes is maintained by the presence of moisture. Upon removal of the moisture from the hydrophilic membrane, the pores undergo large capillary stresses which can lead to shrinkage and cracking of the membrane. Since the membranes have pores of various sizes the capillary stresses formed during drying result in differential stresses throughout the membrane microstructure. This differential stress is known to cause cracks or xe2x80x9cdefectsxe2x80x9d in the membrane. If such a membrane is used to dehydrate a closed system, the moisture in the membrane will be eventually stripped out. This results in the creation of cracks or xe2x80x9cdefectsxe2x80x9d as described above. These xe2x80x9cdefectsxe2x80x9d will now cause the hydraulic transport of oil through the membrane.
U.S. Pat. No. 5,182,022 to Pasternak et al discloses a pervaporation process for the dehydration of ethylene glycol. The ethylene glycol is completely miscible with water, and is characteristic of pervaporation applications where the mixtures to be separated are fully miscible. The sulfonated polyethylene resin membrane that is used permits substantial quantities of ethylene glycol to permeate. It will be apparent to those skilled in the art that the permeation of such quantities of ethylene glycol is due to hydraulic permeation through.defects (see definition below), which are present in the discriminating layer. The invention does not require a defect-free discriminating layer because the loss of the non-aqueous phase is tolerable. This is not the case in the dehydration of oil in a lubrication and hydraulic system.
U.S. Pat. No. 5,464,540 to Friesen discloses a process for the removal of a component from a liquid feed mixture via the process of pervaporation. The sweep stream in the Friesen et al patent is comprised of a component of the feed stream that is not to be removed and is introduced to the module as a vapor. In column 5, lines 8 to 13, Friesen et al postulates that the process can be used to dehydrate oils such as sesame oil and corn oil. However, in the examples provided in the patent, Friesen et al only provides performance data for the dehydration of organic compounds of high volatility, much in excess of sesame oil and corn oil. In particular, Friesen provides examples for the dehydration of acetone, toluene, and ethanol. Consequently, it is clear that Friesen fails to recognize and teach the need for a defect free (as described hereinbelow) non-porous membrane for the dehydration of these types of oils. Those skilled in the art may also question the feasibility of providing a sweep stream of corn oil or sesame oil vapor.
U.S. Pat. No. 5,552,023 to Zhou discloses a membrane distillation technique for the dehydration of ethylene glycol. This process employs a porous membrane. This is unattractive for the dehydration of oils because of the likelihood that the porous support will get wetted out and hydraulically permeate the fluids.
U.S. Pat. No. 6,001,257 to Bratton et al discloses a zeolite membrane that is substantially defect-free for the purpose of dehydration of various liquids. As noted in column 4, lines 12-15 of Bratton, the use of the zeolite membrane is critical to the function of the apparatus, as it can be used to separate any two liquids where only one liquid can pass through the zeolite membrane. Zeolite membranes use zeolitic-type materials, which are also known as molecular sieves, and contain a network of channels formed from silicon/oxygen tetrahedrons joined through the oxygen atoms. Column 2, lines 46-49, indicate that the material should be xe2x80x9csubstantially free of defectsxe2x80x9d, without defining the extent of xe2x80x9csubstantiallyxe2x80x9d or the implied meaning of xe2x80x9cdefectxe2x80x9d. Such a membrane cannot be used for the dehydration of oils because the presence of defects, described hereinbelow, will result in the hydraulic permeation of oil to the permeate side.
In the context of the present invention, the following terms, as used throughout the application, are intended to convey the meanings defined hereinbelow:
xe2x80x9cDefectxe2x80x9d, as used herein, is used to indicate an aperture through the membrane of sufficient size to allow hydraulic permeation of the liquid of low volatility through the membrane.
xe2x80x9cDefect freexe2x80x9d, therefore, indicates a membrane containing no apertures of sufficient magnitude to allow hydraulic permeation of liquids through the membrane, instead limiting the passage of materials through the membrane to solution diffusion. Hydraulic permeation of oil will tend to occur when permanent apertures (i.e. pinholes) of a diameter greater than or equal to the molecular size of oil are present in a membrane. It is expected that the molecular size of the oil molecules is greater than 5 to 10 Angstroms, however since oil consists of fractions of different molecular size, the exact value will depend on the chemical makeup of the particular oil being dehydrated. Thus defect free membranes are limited to apertures of a smaller diameter than the molecular size of the oil molecules.
xe2x80x9cNon-porousxe2x80x9d indicates membranes that do not contain what are commonly referred to as pores, that is permanent apertures of at least the molecular size of the oil molecules, which as discussed above is expected to be greater than 5 to 10 Angstroms, but absolutely dependent on the particular type of oil being dehydrated.
While a defect free membrane, as used herein, is inevitably non-porous, a non-porous membrane, as used herein, is not necessarily defect free. In theory, a non-porous membrane would be one that is defect free, i.e. free from defects as described above. This implies that, a defect free membrane would have the same gas permeability/selectivity as a dense film made from the same material. In practice, however, this is not the case. For example, Pinnau and Koros (Pinnau, I. And Koros, W., xe2x80x9cGas-Permeation Properties of Asymmetric Polycarbonate, Polyestercarbonate, and Fluorinated Polyimide Membranes Prepared by the Generalized Dry-Wet Phase Inversion Process,xe2x80x9d J. Applied Polymer Science Vol. 46 1195-1204 (1992)) and Pesek (Pesek, S. xe2x80x9cAqueous Quenched Asymmetric Polysulfone Flat Sheet and Hollow Fiber Membranes Prepared by Dry/Wet Phase Separationxe2x80x9d Dissertation submitted to The University of Texas at Austin (1993)) have defined a defect-free gas separation membrane as a membrane that has 75% to 85% of the perselectivity of a dense film. It can be shown that, a membrane that has 85% of the permselectivity can contain a significant number of defects that would allow for the hydraulic permeation of oil.
Consider a membrane consisting of a polysulfone selective layer supported by a substructure of negligible resistance. At 35xc2x0 C., polysulfone has an oxygen permeability of 1.4 barrer (Membrane Handbook) and an O2/N2 selectivity of 5.6. Consider the thickness of the polysulfone selective layer to be 700 xc3x85. This thickness is typical for commercially available membranes. Accordingly, the permeance of this selective layer for oxygen would be 20 GPU and for nitrogen 3.57 GPU. According to Pinnau and Koros (1992) this polysulfone membrane would be considered defect free if the O2/N2 selectivity was 85% of the dense film, or in this case 4.76. Obviously according to the definition of the present invention, this membrane contains defects. If the defects are small enough, the flow through the defects will be governed by Knudsen diffusion. If the defects are large, then flow through the defects will be convective (or viscous) and will obey the Hagen-Poiseuille law. The table below illustrates the number of defects of different size that would result in a O2/N2 selectivity of 4.76 for a 1 square meter polysulfone module.
The average size of the defects listed in the above table are large enough to allow hydraulic permeation of oil through the defects and render a oil dehydration module commercially unviable. However, for an application such as gas separation, the presence of the defects merely reduces the efficiency of the separation but does not render the module commercially unviable.
In theory, a non-porous membrane would be one that is defect free, i.e. free from defects as described above. In practice, however, this is not the case. As practiced, and as recognized by one skilled in the art, a membrane that is regarded as being non-porous will allow hydraulic permeation up to a certain factor, typically sufficient to reduce its gas selectivity by up to 85% from the intrinsic selectivity of the dense film, and will still be considered a non-porous membrane. Thus, such a membrane would actually have a relatively small but still significant number of pores. The actual number of pores that would be acceptable in a xe2x80x9cnon-porousxe2x80x9d membrane would be related to the size of the pores and the properties of the materials being separated by the membranes. As used herein, the defect free membranes refer to non-porous membranes that are non-porous as defined hereinabove, and not non-porous as the term is generally used in the art. For the successful practice of the present invention, the membrane must be xe2x80x9cnon-porousxe2x80x9d and xe2x80x9cdefect-freexe2x80x9d as the terms are defined herein.
xe2x80x9cOilxe2x80x9d is used to indicate a low volatility chemical material. Typically, the oil will comprise many fractions of different molecular weight and molecular structure in a mixture.
xe2x80x9cSemi-permeablexe2x80x9d indicates a membrane that allows permeation of certain materials while being resistant to the transport of other materials. Such a membrane can also be referred to as a discerning membrane.
xe2x80x9cWettingxe2x80x9d indicates the spreading of a liquid over a surface.
xe2x80x9cFoulingxe2x80x9d indicates adding a resistance to mass transfer through an undesirable action such as filling the porous substructure of the membrane with oil, or coating the sweep side of the membrane with oil.
The present invention provides a membrane based process for removing free, emulsified or dissolved water from oils or other liquids of low volatility. This process is such that it may be used on mobile equipment while in operation and moving, as well as on stationary equipment and processes. The operation of this process is simple, while the equipment in question is small and compact making it practical and cost effective for systems of all sizes.
The present invention further provides for a defect-free discriminating layer, or membrane, which does not permit the hydraulic permeation of liquids through it, restricting permeation to transport through the discriminating layer. The invention further provides for the removal of the vapors permeating through the discriminating layer. Thus, the present invention provides an apparatus and method for more efficiently separating free, emulsified and dissolved water from oil.
Specifically, this invention relates to the process of using a non-porous, defect free membrane to remove water selectively from oils. More particularly, the process consists of removing water from the oil stream of concern by contacting the oil with one side (xe2x80x9cfeed sidexe2x80x9d) of a semi-permeable membrane. The membrane divides a separation chamber into the feed side into which the oil is fed, and a permeate side from which the water is removed. The permeate side is maintained at a low partial pressure of water through presence of vacuum, or by use of a sweep gas. The water in the oil may be either in the dissolved form, or, as a separate phase, either emulsified, dispersed or xe2x80x9cfree.xe2x80x9d The membrane material is one that is of the appropriate chemical compatibility with the oil, while selectively permitting the transport of water across it. The membrane is chemically compatible with the oil if it does not chemically react with the oil, or if its physical properties such as size, strength, permeability, and selectivity are not adversely affected by contact with the oil.
Thus, one of the objects of the present invention is to overcome the shortcomings of conventional oil dehydration techniques, and provide a new apparatus and a process for dehydrating oil that overcomes these limitations.
Another object of this invention is to provide an oil dehydrator that removes free, emulsified or dissolved water from oils.
A further object of the present invention is to provide an oil dehydrator that is simple to operate.
A further object of the present invention is to provide an oil dehydrator that is relatively small and compact.
A further object of the present invention is to provide an oil dehydrator that is cost effective.
A further object of the present invention is to provide an oil dehydrator that is practical to use on small and large systems.
A further object of the present invention is to provide an oil dehydrator that may be used on mobile equipment while in operation and moving.
Further objects and advantages of the present invention will be apparent from the following description and appended claims, reference being made to the accompanying drawings forming a part of the specification, wherein like reference characters designate corresponding parts in the several views.